1. Field of the Invention
The present invention relates to an endoscopic image processing system. More particularly, this invention is concerned with an endoscopic image processing system including a shape estimating unit for estimating the absolute shape of an object entity according to images of the same object entity produced at different positions.
2. Description of the Related Art
Assume that a plurality of imaging means are arranged so that the fields of view offered by the imaging means will overlap and that the relative locations of the imaging means are known. Under the circumstances, the question of how to estimate the shape of an object according to produced images is treated as the question of estimating a shape from so-called stereoscopic images. Various methods have been proposed for overcoming the question.
In recent years, some proposals have been made for a method of extracting a three-dimensional structure from motions. This method is construed as a technique attempting to estimate relative movements of an imaging means using a plurality of images.
The methods employ as raw information association of the positions of the same point on an object appearing in a plurality of images.
Various methods have been proposed of the way of detecting the positions of the same point on an object appearing in images. In the case of artificial objects, components of angles and a contour are often apparent. A technique of extracting a structure including line segment extraction would prove effective. However, there is difficulty in adapting the technique to ordinary natural images. Density gradient is a technique often adapted to time-sequential images. This technique provides good results when an apparent movement of an object observed between images is very small and image quality is good. However, many restrictions are imposed on the conditions for imaging.
In general, therefore, an area surrounding a point concerned in an image is extracted as a reference area. The reference area is correlated with an area extracted from an image of an object of search. A position giving a maximum correlation value is regarded as a corresponding point. This technique (block matching) is often adopted. Block matching provides relatively stable results as long as the texture of an image is clear. If the texture is unclear, an incorrect corresponding point may be detected.
Furthermore, a primary drawback of the block matching technique lies in that when an object is three-dimensional and a boundary relative to a background is contained in a block, the results of detection are unreliable. Moreover, the surface of an object may be inclined relative to an imaging means or the distances from a plurality of imaging means to the object may be mutually different. Images produced by the plurality of imaging means may visualize the object as a substantially deformed object. Also, there may be large differences in size among the images. In any of these cases, the results of detection are unreliable.
A question concerning estimation of a three-dimensional shape is the question of occlusion or of matching the results of estimation. In particular, when stereoscopic images are used for estimation, a portion of an object may be hidden and therefore not be imaged or a portion thereof may be imaged by only one imaging means. This the question of how to treat these arises.
If the number of imaging means is large, the number of areas to be imaged becomes accordingly large. The number of hidden areas is limited. However, when the positions of imaging means are unknown or estimation is not carried out accurately, it is not easy to match images.
Not almost all conventionally proposed techniques can be said to be fully satisfactory in terms of practice abilities. This is because the techniques are concerned with images of an artificial object. These techniques attempt to address the foregoing questions, which arise without fail when a natural object is imaged, on the presumption or condition that influence is ignored or underestimated.
For example, images produced by a biomedical endoscope are images for which it is hard to adapt the conventionally proposed techniques. However, if the techniques properly functioned on the images, they would prove very valuable in practice.
Endoscopes have an elongated insertion unit which is inserted into a body cavity, whereby a lesion or the like in the body cavity can be observed without incision or can be cured or treated using, if necessary, a treatment appliance. The size of the distal part of an endoscope must be minimized from a functional viewpoint. Any members other than those required by a doctor for observation or treatment cannot therefore be incorporated in the distal part.
Some proposals have already been made of grasping the shape of an object trans-endoscopically. Proposed are, for example, a method of projecting patterned light or the like on an object of observation (Japanese Unexamined Patent Publication No. 63-240831), and a system having a compound eye incorporated in the distal part of an endoscope (Japanese Unexamined Patent Publication No. 63-244011). In either case, a special structure must be formed in the distal part of an endoscope or in a light source unit. This leads to a large or complex endoscopic system. In general, the method and system can therefore not be easily adopted.
Japanese Unexamined Patent Publication No. 63-246716 filed by the same applicant has disclosed a technique of estimating the shape of an object using a plurality of images produced by moving the distal part of an endoscope through manipulations. Also disclosed is a measuring mechanism for measuring magnitude of shifts made by the distal part responsively to manipulations. According to this technique, the absolute shape of an object can be estimated without impairment of the functions of an existing endoscope.
Moreover, Japanese Unexamined Patent Publication No. 6-7289 filed by the same applicant has disclosed a technique of estimating the relative shapes of an object and reference entity using a plurality of images produced by moving the distal part of an endoscope through manipulations. Also disclosed is a technique of estimating the size of an object according to the size of a reference entity located near the object.
According to the constituent features of the foregoing Japanese Unexamined Patent Publication No. 63-246716, the relative shapes of an object entity are estimated using a plurality of images produced time-sequentially. Magnitudes of shift made by an imaging means incorporated in the distal part of an endoscope responsively to the manipulations are measured. The absolute shape of the object is then estimated based on the measured value.
However, precision offered by the measuring mechanism is neither insufficient nor sufficient in terms of the resolution an imaging means is desired to provide.
Moreover, according to the constituent features of the Japanese Unexamined Patent Publication No. 6-7289, when an object entity is imaged time-sequentially, a reference entity having a size is known is imaged concurrently with the object entity. The absolute shape of the object entity is thus drawn out.
However, when a biomedical endoscope is used to concurrently image the object entity and the reference entity placed near the object entity, an operator of the endoscope has a large burden.
Japanese Unexamined Patent Publication No. 9-26547 has proposed an endoscopic system capable of precisely estimating an absolute distance from an imaging means to an object entity using images of the same object entity produced at different positions.
However, in the Japanese Unexamined Patent Publication No. 9-26547, the relative shapes are estimated using images of the object entity produced at different positions. Images exhibiting halation or images produced when laser light has been irradiated are used to calculate a distance between the distal part of an endoscope and an object and to thus estimate the absolute shape of the object. This poses a problem in that precision varies depending on the states of images employed in the calculation of a distance.